Composite underground storage tank: a steel primary tank with a nonmetallic external cladding or laminate constructed of fiberglass and polyester or urethane resin. These are atmospheric steel tanks intended for storage underground of flammable and combustible liquids. Threaded fittings / openings can be located along the tank's top centerline or openings can be installed inside containment sumps. Glasteel tanks may be single or double wall construction, with or without multiple compartments for multiple fuel storage. If the tank is a double wall configuration, then it has interstitial monitoring capabilities. Glasteel tanks are built to Underwriters Laboratories UL-58, UL-1746 and Steel Tank Institute's ACT-100 specifications.